Stabilized servo mechanism



Dec. 14, 1954 D. E. KENDALL. JR

STABILIZED SERVO MECHANISM Filed Nov. 9, 1951 Dell;

[nvenzar n E'Jfendall Jr United States Patent STABILIZED SERVO MECHANISM Delvin E. Kendall, .lr., Weston, Mass., assignor to United Shoe Machinery Corporation, Flemington, N. J., a corporation of New Jersey Application November 9, 1951, Serial No. 255,653

4 Claims. (Cl. 121-38) This invention relates to servo mechanisms, and has for its principal object the provision of a novel and improved servo mechanism of the closed loop type in which the operation of a servomotor is controlled by a device having a movable member the displacement of which directly determines the direction and speed of movement of the motor.

In servo mechanisms of the aforementioned type, the motor may be a fluid-pressure-operated piston contained within a power cylinder while the controlling device may consist of a valve, or fluid jet relay, which is connected to the opposite ends of the power cylinder by suitable conduits. For example, in an application for United States Letters Patent Serial No. 279,084, filed March 28, 1952, in the names of S. J. Finn, Walter L. Benedict, Hans F. Schaefer, Jr., and John W. Hursh, there are disclosed six servo mechanisms of this type in each of which the movements of a fluid-pressureoperated piston are controlled by a fluid jet relay comprising a movable jet nozzle for controlling the flow of pressure fluid to and exhaust from the opposite ends of the power cylinder in which the piston is contained. In the apparatus disclosed in the mentioned application, each movable jet nozzle is actuated by a feeler, which directly engages a surface of the shoe, the position of which is determined by the associated piston, so that a closed loop servo mechanism is provided. The fluid jet relays which form the controlling devices of each of these closed loop servo mechanisms are of a novel and improved construction disclosed and claimed in United States Letters Patent No. 2,672,150, granted March 16, 1954, in the names of Walter L. Benedict and Sidney J. Finn, which as is explained ,in that patent, have a very high degree of operating sensitivity. Accordingly, and due to the relative incompressibility of the pressure fluid such, for example, as oil, where the movable jet nozzle is subject to high frequency oscillations during the operation of the servo mechanism, the piston of the servomotor will tend to vibrate at a correspondingly high rate, in response to rapid pressure impulses delivered from the jet relay to the power cylinder.

To avoid such undesired vibrations of the servomotor piston the herein illustrated servo mechanism is, in accordance with a feature of this invention, provided with means for absorbing high-frequency pressure impulses, set up by the controlling device, prior to response thereto by the servomotor. The herein illustrated servo mechanism includes a servo motor comprising a piston mounted within a power cylinder, a source of fluid under pressure, a fluid jet relay for controlling the flow of pressure fluid to and exhaust from the opposite ends of the power cylinder, a pair of conduits connecting the discharge passages of the fluid jet relay to the opposite ends of the power cylinder, and. associated with these conduits, is the means for absorbing high-frequency pressure impulses set up by the jet relay. More particularly, the means for absorbing the high-frequency pressure impulses comprises a flexible diaphragm and a restricted passageway interposed in a by-pass conduit, or pipe, extending between the aforementioned conduits. With this arrangement, the servomotor piston responds directly to displacements of the jet nozzle of the relay in the usual manner so long as the movements of the nozzle are of a relatively low frequency. However, when the frequency of movement of the jet nozzle is increased beyond a predetermined minimum value, de-

2,696,804 Patented Dec. 14, 1954 Ice pending on the size and stiffness of the diaphragm, the size of the restricted passageway in the by-pass pipe, the pressure of the operating fluid, the mass of the servomotor piston and parts connected thereto, etc., the resulting high-frequency pressure impulses, set up in the conduits leading from the relay to the power cylinder, will be absorbed prior to response thereto by the piston of the servomotor.

The above and other objects and features of the invention will appear in the following detailed description of the preferred embodiment thereof, illustrated in the accompanying drawings, and will be pointed out in the claims.

In the drawings,

Fig. 1 is a schematic perspective view of a closed loop servo mechanism embodying the features of this invention; and

Fig. 2 is a similar view, at an enlarged scale and with certain parts broken away and others shown in section, of a portion of the servo mechanism illustrated in Fig. 1.

Referring to these drawings, the servo mechanism illustrated schematically in Fig. 1 corresponds to one of the servo mechanisms disclosed in the aforementioned Finn et al. application which, as is explained in that application, is arranged to control the heightwise position of a shoe, relatively to operating instrumentalities, not here shown. Thus this servo mechanism includes a servomotor comprising a piston 10 contained within a power cylinder 12; a jet relay indicated generally by the reference character 14 and having a flexible jet pipe 16; and a shoe-engaging feeler 18 directly connected to the flexible jet pipe by means of a link 20, The jet pipe 16 extends downwardly from a header block 22, which is supported on a base member 24 by means of a pair of pipes 26, 26, and carries on its lower end a jet nozzle 28. This nozzle is positioned closely adjacent to the upper surface of the base member and directly over a pair of openings 30, 32 which are associated with the base member and connected, respectively, to a pair of discharge passages 34, 36. Fluid under pressure from a source, not shown, is supplied to the jet relay through an inlet pipe 38 which is connected to the jet pipe 16, and hence to the jet nozzle 28, by means of passageways 40, 42 in the base member, the pipes 26, 26 and passageways 44, 46 in the header block 22. As will be readily understood from reference to the aforementioned Benedict et al. patent, this jet relay would be enclosed within a suitable housing member, provided with a discharge passageway for conducting exhausting pressure fluid back to a sump. However, to simplify the present disclosure, this housing and the discharge passageway have been omitted from Fig. 1. The discharge passageways 34, 36 in the base member 24 are connected to the opposite ends of the power cylinder 12 by means of a pair of conduits 50, 52.

The piston 10 is provided with a piston rod 60 on which there is mounted a shoe holder, indicated generally by the reference character 62 and having a last pin 64 and a toe support 66. As is explained in detail in the aforementioned Finn et al. application, the cylinder 12 is associated with a jack mechanism, not here shown, which is arranged to effect rectilinear and rotational movements of the shoe holder in such a way as to transfer a point of operation around the marginal portion of the bottom of a shoe S supported on the holder. During such operation of the jack mechanism, the bottom of the shoe, at the point of operation, is maintamed in a predetermined heightwise position, relatively to the operating instrumentalities, not shown, by vertical movements of the shoe holder 62 which are effected by the piston 10. Such vertical movements of this piston are controlled by the jet relay 14, which is operated by the feeler 18, in the following manner.

The feeler 18 is formed on one end of a bell-crank lever 70 which is pivotally mounted on a shaft 72, supported in suitable bearings, not shown, and is directly connected to the jet nozzle 28 by the link 20. The flexible jet pipe 16 is so positioned as to maintain a discharge orifice, not shown, in the jet nozzle 28 centered 'with respect to the openings 30, 32, which, as is explained in the aforementioned Benedict et al. patent, are

symmetrically disposed on the opposite sidesof a vertical plane that is substantially perpendicular to the link 2t). Preferably, and as disclosed in the aforementioned Benedict et al. patent, the action of the flexible pipe 16 will be supplemented by a spring-centering mechanism, not herein shown. Connected to the lever 70 is a coil spring 74 which. tends, at all times, to displace the jet nozzle 23 to the left, as viewed in Fig. 1, against the resistance of the flexible pipe 16 and, of course, of the centering spring arrangement, if provided.

The servo mechanism described above operates in the following manner. As the jack, not shown,. is operated to effect rectilinear and rotational movements of the shoe holder 62, so as to transfer a point of operation along the marginal portion of the bottom of the shoe S, in the manner explained in detail in the above-mentioned Finn et al. application, the feeler 18 is held in engagement with the bottom surface of the insole I, adjacent to the insole rib R, by the spring 74. The rectilinear and rotational movements of the shoe holder take place in a plane generally parallel to the upper surface of the holder 62. Accordingly, and due to the longitudinal curvature of the bottom of the shoe, the bottom of the shoe at the operating point adjacent to the marginal edge thereof would tend to depart fromthe desired heightwise position, relatively to the operating instrumentalities, not shown. However, such departure is immediately detected by the feeler 18 which displaces the jet nozzle 28 relatively to the openings 30, 32, and thereby causes the piston 10 to effect vertical movement of the shoe holder and thus restore the operating point on the bottom of the shoe to the desired heightwise position. For example, if the bottom of the shoe curves downwardly, the lever 70 will be swung in a counterclockwise direction by the spring 74 and the jet nozzle 28 will be displaced to the left, Fig. l. The pressure of the fluid in the conduit 52 will now be increased while the pressure of the fluid in the conduit 50 will be decreased, thus causing an upward movement of the piston 10 which is continued until the jet nozzle 28 is returned to its centered position, relatively to the openings 30, 32. Conversely, if the bottom of the shoe curves upwardly, the feeler 18 will be elevated, thus swinging the lever 70 in a. clockwise direction and displacing the jet nozzle to the right, Fig. 1. Now the pressure of the fluid in the conduit 50 will be increased while the pressure of the fluid in the conduit 52 is decreased, thereby causing the piston 10 to move the shoe holder downwardly. As the shoe holder is thus lowered, the feeler 10 will be swung downwardly by the spring 74 and the jet nozzle 28 returned to its centered position when the bottom of the shoe, at the point of operation, has been brought to the desired heightwise position, relatively to the operating instrumentalities, not shown.

As is explained in the aforementioned Benedict et al. patent, the jet relay herein illustrated is extremely sensitive in operation so that the pressure of the operating fluid in the two conduits t), 52 will be materially unbalanced by extremely small displacements of the jet nozzle 28, for example, in the magnitude of a few thousandths of an inch. operating fluid used, e. g., oil, the pistonv will tend to respond, by movement in the cylinder 12, to the slightest unbalancing of the pressure of the operating fluid in the conduits 50, 52, produced by displacement of the jet nozzle 23 from a centered position, relatively to the openings 39, 32 in the base member 24. As the shoe S is fed along by means of the jack mechanism, the feeler 18 may, at times, be oscillated at a rapid rate by riding over relatively small irregularities of the surface of the insole I or by other causes such, for example, as the nature of the curvature of the shoe bottom or because of vibrations of the shoe holder by external forces imposed thereon by instrumentalities operating on the shoe. As a result of such rapid oscillation of the feeler 18, and of the jet nozzle 28 which is directly connected thereto by the link 20, a rapid series of pressure impulses will be set up alternately in the conduits S0, 52 and the piston 10 will tend to respond to these pressure impulses and to impart a rapid vibratory movement of the shoe holder 62. Such rapid vibratory movements of the shoe holder and of the shoe carried thereby are extremely undesirable, particularly from the standpoint of satisfactory operation along the marginal portion of the bottom of the shoe. Also, under certain. conditions, the servo mechanism Due to the relative incompressibility of. the.

may tend to hunt, and even to become unstable, in response to input signals, displacements of the jet nozzle of a frequency exceeding a predetermined minimum value.

For the purpose of eliminating such undesirable rapid vibrations of the shoe holder and thus stabilizing the operation of the servo mechanism, the herein illustrated mechanism is provided with the following arrangement. Extending between the two conduits 5t), 52 in a location closely adjacent to the jet relay 14, is a by-pass compris' ing two pipes 80, 82 and a hollow housing 84, see Fig. 2. Associated with this housing is a thin metallic plate, or diaphragm 86, which prevents the direct flow of pressure fluid between the two pipes 80, 82, while fitted within the pipe 32 is a bushing 88 provided with a bore 9i of considerably smaller diameter than that of the pipe 32. With this arrangement, when the size and flexibility (i. e., spring factor) of the diaphragm 86 and the diameter of the restricted passageway provided by the bore in the bushing 88 are suitably selected with regard to the pressure of the operating fluid, the size and length of the conduits 59, S2 and the mass of the piston 10 and of the parts moved by that piston, high-frequency pressure impulses, which may be set up in the conduits 5Q, 52 as a result of the rapid vibration of the feeler 18 and the correspondingly rapid movement of the jet nozzle 28, in the manner explained above, will be absorbed by the action of the diaphragm 86 and restricted passageway 90 before the piston lit) can respond thereto. Thus, the piston it? still responds directly to all displacements of the jet nozzle which are imparted to it by the feeler, so long as movements of the jet nozzle are of a relatively low frequency, so that the shoe holder 62 is moved vertically by this piston, in response to the action of the feeler 18, to maintain the bottom of the shoe at the point of operation in a desired heightwise position. However, when the frequency of movement of the jet nozzle is increased beyond a predetermined minimum value, the resulting high-frequency pressure impulses, set up in the conduits 5t), 52 which lead from the jet relay 14 to the cylinder 12, are absorbed prior to response thereto by the piston 10. Accordingly, the servo mechanism operates very smoothly to move the shoe holder up or down, as required by the longitudinal curvature of the bottom of the shoe S. Yet all undesirable rapid vibrations of the shoe holder are avoided.

The action of the diaphragm 86- and restricted passageway 90, which are interposed in the bypass formed by pipes 80, 82, to absorb high-frequency pressure impulses, setup alternately in the conduits 5t), 52 as a result of rapid movement of the jet nozzle, may be briefly described as follows. Because of the inertia of the piston 10, and of the parts which are carried thereby, the responsive movement of this piston lags, more or less, behind the displacement of the jet nozzle. Hence, before the inertia of the piston 10 and of the parts carried thereby is overcome by a pressure impulse, or build up, set up in one of the conduits as a result of a displacement of the jet nozzle, the pressure of the fluid in that conduit will be more or less relieved, by the flow of a small quantity of fluid from the restricted passageway 90, while the pressure of the fluid in the other conduit will be correspondingly increased, by the introduction of a similar quantity of fluid, expelled thereinto by the deflection of the diaphragm 86 in response to the pressure differential between the two conduits. This pressure equalizing action, of course, increases as the intensity of the pressure impulse, and hence the unbalanced force acting on the diaphragm increases, and it is delayed, more or less, by the action of the restricted passageway. Now the intensity of the pressure impulse and also the time delay between pressure build up in one of the conduits and actual movement of the piston, i. e., phase lag, will increase as the frequency of displacement of the jet nozzle increases. Accordingly, when the frequency of nozzle displacement reaches a predetermined value, before the piston 1t) actually begins to move, pressure of the fluid acting on its opposite sides will be substantially balanced so that the piston remains stationary. Thus, the diaphragm 86 and restricted passageway 9ft, in effect, function to absorb high-frequency pressure impulses, above a predetermined minimum value, set up in the conduits 5t), 52 as a result of rapid oscillation of the jet nozzle 28, before the piston 10 can respond thereto and the operation of the servo mechanism is effectively stabilized. However, as the frequency of oscillation of the jet nozzle decreases and the intensity of the pressure impulses, as Well as the magnitude of the phase lag, become smaller, the absorbing action of the diaphragm and restricted passageway become less pronounced until, at low frequencies, below a predetermined value, the absorbing effect practically disappears. Stated another way, the diaphragm and restricted passageway may be considered as functioning in the manner of a hydraulic filter which absorbs, or attenuates pressure impulses, i. e., signals to actuating cylinder, of a frequency above a predetermined value. By thus reducing the so-called gain of the system in response to input signals, i. e., displacements of the jet nozzle, of higher frequencies, the operation of the servo mechanism is stabilized and materially improved, inasmuch as the tendency of the servo piston to hunt, or of the servo mechanism to become unstable, is greatly reduced.

It will be apparent, the exact size and stiffness (spring factor) of the diaphragm 86 and the diameter of the restricted passageway 90 will vary in accordance with the physical characteristics of the servo loop, i. e., the pressure of the operating fluid, the size and length of the conduits 50, 52,-and the mass of the piston 10 and of the parts carried thereby. While it is, of course, possible to develop mathematical formulae for use in computing the size and stiffness of the diaphragm and the diameter of the restricted passageway which are required for stabilizing a servo loop of known physical characteristics, it has been found that these values can usually be determined more readily by experimentation. To facilitate such procedure, an adjustable needle valve can be utilized in one of the pipes 80, 82, in place of the bushing 88 with its restricted passageway 90 of fixed diameter, and a number of diaphragms 86 of different spring factor and/or diameter, substituted in appropriately sized housings 84 until, by adjustment of the needle valve, the desired pressure impulse absorbing action is obtained with the particular servo loop being used.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent of the United States is:

1. A servo mechanism comprising, in combination, a fluid-pressure-operated motor, a pair of conduits connected, respectively, to the opposite ends of said motor, a source of fluid under pressure, a device for controlling the flow of fluid under pressure from said source to, and exhaust from, said conduits including a movable member, and means including a diaphragm and a restricted passage connected between said conduits for absorbing highfrequency pressure impulses, set up in said conduits as a result of the rapid oscillation of said movable member, prior to response thereto by said fluid-pressure-operated motor.

2. A servo mechanism comprising, in combination, a fluid-pressure-operated motor including a cylinder and a piston, a pair of conduits connected, respectively, to the opposite ends of said cylinder, a source of fluid under pressure, a device for controlling the flow of fluid under pressure from said source to, and exhaust from, said conduits including a movable member, and means, including a diaphragm and a restricted passage connected between said conduits, for absorbing high-frequency pressure impulses, set up in said conduits as a result of rapid oscillation of said movable member, prior to response thereto by said piston.

3. A servo mechanism comprising, in combination, a fluid-pressure-operated motor, a pair of conduits connected, respectively, to the opposite ends of said motor, a source of fluid under pressure, a fluid jet relay for controlling the flow of fluid under pressure from said source to, and exahust from, said conduits including a movable jet nozzle, and means including a diaphragm and a restricted passage connected between said conduits for absorbing high-frequency pressure impulses, set up in said conduits as a result of rapid oscillation of said movable jet nozzle, prior to response thereto by said motor.

4. A servo mechanism comprising, in combination, a fluid-pressure-operated motor including a cylinder and a piston, a pair of conduits connected, respectively, to the opposite ends of said cylinder, a source of fluid under pressure, a fluid jet relay for controlling the flow of fluid under pressure from said source to, and exhaust from, said conduits including a movable jet nozzle, and means, including a diaphragm and a restricted passage connected between said conduits, for absorbing high-frequency pressure impulses, set up in said conduits as a result of rapid oscillation of said movable jet nozzle, prior to response thereto by said piston.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,247,301 Lesser June 24, 1941 FOREIGN PATENTS Number Country Date 487,680 Great Britain June 20. 193R 

